1. Field of the Invention
The present invention relates to microgyroscopes, and in particular to vibratory microgyroscopes. More particularly, this invention relates to techniques for operating nearly-symmetric microgyroscopes or inertial wave microgyroscopes.
2. Description of the Related Art
Micromachining has brought compact, low-cost, low-power vibratory gyroscopes for safety and global positioning system (GPS)-aided navigation applications. However, their performance so far has been unsatisfactory for inertial positioning, pointing or autonomous navigation.
Symmetric degenerate mode vibratory gyroscopes present definite advantages for inertial navigation applications. They have been demonstrated to be capable of high performance and enable improved manufacturability and low cost when properly designed. The challenge of delivering high performance lies in achieving very high and closely-matched resonator Q's for both degenerate modes. This goal has been difficult to achieve for small devices capable of being mass produced.
In the late 1800's G. H. Bryan first identified the opportunity for a vibratory gyroscope based on ideal inertial wave operation. He understood that in a rotating, axisymmetric elastic continuum, a transverse traveling wave could be excited to propagate analogous to the inertial waves. This also applied in a rotating axisymmetric fluid continuum. Both waves were a balance between dynamic inertial, or Coriolis forces and elastic or pressure gradient forces. Both transverse waves propagated or precessed in the rotating frame at a precise fraction, k, of the inertial rate. The fraction, k, or angular gian is determined only by the geometric shape of the continuum, not the dimensions. Further, he identified the hemispherical shape as having a Coriolis-coupled vibratory mode with useful finite angular gain k=0.3 and deduced that it could be readily used to determine an inertial rate of rotation, Ω, by simply measuring the inertial wave precession rate, Ωp, and dividing by k, i.e. Ω=Ωp/k. Typically modal electrical phase, Ωpm=nΩp is measured rather than the actual mechanical precession phase and for a hemisphere, cylinder or disc shape, the Coriolis-coupled vibratory mode with n=2 is often selected. The extreme stability of the shape (sub parts per million), and hence the rate scale factor, has not yet been achieved in practice by any vibratory gyroscope even today.
Limitations in design, fabricated mechanical precision and quality and subsequent electronics operation have further resulted in high drift and noise that hinder the achievement of ideal inertial wave-based operation and performance. Ideally, a vibratory gyroscope should have drift and noise limited only by random physical noise i.e., thermal mechanical noise and/or random white electronics sensor noise in the very narrow vicinity of the resonator frequency. Such random physical noise can be mitigated using higher mechanical Q, mass and vibration amplitudes and higher capacitive sense area to maximize signal to electronics noise and sensitive electronics circuits. Current vibratory gyroscopes can generally be placed in two classes, asymmetric designs with a closed loop drive and un-tuned open-loop output, e.g. a tuning fork type, and axisymmetric designs with a closed loop drive and tuned closed loop output, e.g. a hemisphere, ring, or cylinder.
Conventionally-machined gyroscopes suitable for inertial wave operation such as the quartz hemispherical resonator gyroscope (HRG) have an ideal axisymmetric design, finite angular gain, k=0.3, with near-ideal mechanical fabrication precision and quality, but are not compact, low-cost and low-power. Furthermore, the HRG electronics operation limits performance. Several key parameters of the vibratory modal motion are not disciplined, e.g., resonator frequency and damping non-uniformity, leading to rate drift over temperature and are permitted to naturally vary with time or temperature or free-run. In some vibratory gyroscope designs the difference in the natural frequencies of the two resonator modes are controlled to zero or disciplined by driving output quadrature voltage to zero by modification of electrostatic biases to modify electrostatic stiffness, (quadrature nulling) or by feedback of the modal motion position states. Failure to discipline all parameters necessitates expensive calibration of the final rate output bias over temperature and case-orientation of the vibration pattern due to changes in the undisciplined parameters. Resonator state feedback is used to track the natural drive frequency and phase and control the amplitude using an automatic gain control (AGC) loop and sometimes the output axis in a force-to-rebalance loop. Sometimes drive frequency and phase is tracked with a phase-lock loop. Further, the output disturbance noise of the closed loop electronics of the HRG design is limiting noise and drift performance. A type of inertial wave operation (i.e., ‘whole-angle’ or ‘rate-integrating’) has been used with the HRG, however the natural frequency and natural damping unbalance are still allowed to freely change with temperature and time. Case-fixed closed loop operation or free-precession operation of the HRG, at very low inertial rates, does not offer the opportunity to completely identify changes in the stiffness and damping parameters of motion in all directions.
On the other hand, micromachined gyroscopes, with less ideal designs, mechanical precision and quality, suffer similar performance limitations, but to a much larger degree. Some of these designs, e.g., ring resonators, have employed a type of inertial wave operation for very high rate applications, but lack navigation grade sensitivity at very low input rates.
The result of this state of the art is that, as improved micromachined designs are advanced with improving mechanical precision and quality, the potential performance of inertial wave operation with low noise and drift corrected to the limits of the physical mechanical and sensor noise has not been achieved. This is due to the failure to complete the parametric discipline of a nearly-ideal inertial wave gyroscope.
In particular, the resonator frequency parameter and the non-uniform damping parameters have not before been disciplined in the operation of current small vibratory gyroscopes. In addition, the use of state-feedback in the prior art limits the performance of current gyroscope designs. Furthermore, digital electronics in a closed loop control consume additional power as sample frequency and computational precision are increased to improve accuracy and dynamic range.
In view of the foregoing, there is a need in the art for gyroscopes, particularly small vibratory gyroscopes such as an inertial wave gyroscope, and their methods of operation to have improved performance (e.g., higher and more closely matched Q, lower drift and lower noise) for navigation and/or spacecraft payload pointing. There is a need for such gyroscopes to operate under disciplined control of frequency and/or damping. There is also a need for gyroscope control electronics to operate with reduced power and computational requirements. As detailed hereafter, the present invention satisfies these and other needs.